Trilobites: Turn to the left - Turn to the right

[piranha]

text and figures from:

 

Zong, R.W., & Gong, Y.M. (2017)

Behavioural asymmetry in Devonian trilobites.

Palaeogeography, Palaeoclimatology, Palaeoecology, 476:158-162  LINK

 

 

Trilobites, like many extant arthropods, were in a soft-shell state without defensive ability just after moulting (Henningsmoen, 1975). It is possible that Omegops cornelius and Plagiolaria nandanensis would choose to get away from the discarded cephala to avoid any potential harm that might ensue by touching the hard old cephala. They were likely to turn left (counterclockwise) to move away if the old cephalon was discarded on the right side of the body, or otherwise to turn right (clockwise) if the old cephalon was on the left side. Therefore, the asymmetric distribution of cephala during the moulting process reflects the asymmetric motion in these two phacopid genera. Specifically, Omegops cornelius preferred counterclockwise motion, so that it would choose to push the old cephalon to the right side of the body (Fig. 3A), whereas Plagiolaria nandanensis preferred clockwise motion (Fig. 3B).

 

As mentioned above, the behavioural asymmetry of two trilobites are opposite. It may be helpful in tracing the origin of the asymmetry. One plausible explanation is connected with their body structures, in which the most obvious difference is the size of their eyes; does the large-eyed Omegops cornelius prefer the counterclockwise motion, and the small-eyed Plagiolaria nandanensis prefer the clockwise motion? If so, the preferred direction of movement of Omegops cornelius and Plagiolaria nandanensis found in other areas should be the same as is revealed here.

 

The statistical evaluation of the cephalon position relative to the trunk reflects that Omegops cornelius from the Upper Devonian of the western Junggar (Xinjiang), preferred to turn the cephalon to the right side of the body during the moulting process, showing a preference of counterclockwise motion, whereas Plagiolaria nandanensis from the Lower Devonian of Nandan (Guangxi) preferred clockwise motion. This phenomenon demonstrates that behavioural asymmetry existed in Phacopid trilobites, but may be opposite in different trilobites. The origin of such asymmetric behaviour is unclear, two possible explanations could be related to the body structures of trilobites, and the environment where trilobites lived in the specific geologic age.

 

[Kane]

Very interesting. Certainly helps better explain the deposition of moults and orientation. An excellent contribution.

[Malcolmt]

Yikes another project for me to put on my to do list analyzing the direction of head molts on all my Penn Dixie e. rana

[Raggedy Man]

Thanks for posting this! As the previous post referenced Penn Dixie as I was taking this pic, here it is.

Eldredgeops rana

[Fossil-Hound]

@Monica just recovered an example of this.